docs: def tag(ctx, input, output):
      """Output POS-tagged tokens."""
      log.info('chemdataextractor.pos.tag')
      log.info('Reading %s' % input.name)
      doc = Document.from_file(input)
      for element in doc.elements:
          if isinstance(element, Text):
              for sentence in element.sentences:
                  output.write(u' '.join(u'/'.join([token, tag]) for token, tag in sentence.pos_tagged_tokens))
                  output.write(u'\n')